'It would inexcusable not to take the strongest action now.'

Government ministers have known about the adverse effects of diesel cars on people’s health for decades, campaigners say.

Papers shown to the BBC revealed proposals on cutting the number of high-polluting vehicles - which contribute heavily to poor air quality - were mooted back in 2003, when Gordon Brown was chancellor.

But civil servants objected to a stronger policy to deter diesel usage on presentational grounds, because they did not want the government to be seen as “penalising” diesel drivers.

Environmental lawyers ClientEarth described the revelations as “a scandal” and said the government - including current chancellor Philip Hammond, who will deliver his autumn budget next week - must now take proper action.

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Proposals to cut the number of diesel vehicles were first floated when Gordon Brown was chancellor.

CEO James Thornton said: “This is truly unbelievable – an absolute scandal. Ministers knew full well that diesel was damaging people’s health but still ploughed ahead with policy that populated this country’s roads with diesel vehicles.

“Public health has consistently played second fiddle to false political priorities – and ministers are still not acting urgently enough to remedy it. The government cannot bargain with people’s health again.

“We now know that the government has known for decades that diesel was harmful. And we’re still living in a country with illegally poor air quality, despite two court orders against the government, while the tax system continues to favour the very vehicles that keep it dirty.”

Environment secretary Michael Gove wants to phase out petrol and diesel cars completely by 2040, but the government has shied away from implementing a full network of Clean Air Zones in the meantime - which would ban high-pollution vehicles from areas with poor air quality.

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Michael Gove is under pressure to do more to reduce air pollution.

Instead, local councils are being asked to tackle pollution hotspots in their own individual areas - but ClientEarth said the budget offers ministers a second chance to strengthen their policies.

“Michael Gove needs to stop passing the buck to local authorities and actually put the people of this country first,” Thornton added.

“Ministers have knowingly jeopardised our health and they must under absolutely no circumstances let it happen again.

“The chancellor must urgently abolish the incentives that have condemned the country to illegally poor air quality for decades.

“It would be inexcusable not to take the strongest action now.”

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The Help Britain Breathe campaign, which is backed by ClientEarth and the British Lung Foundation, is also lobbying for tougher action on poor air quality, which is responsible for 40,000 premature deaths every year.

British Lung Foundation chief executive Dr Penny Woods said:“Air pollution is a silent and invisible killer and road traffic is the biggest culprit. The government should act immediately by using the budget to end incentives to buy diesel vehicles, and finally commit to a new clean air act.

“The extent of the public health emergency we are facing with the UK’s air quality demands nothing less than urgent action to protect our lung health.”

Gove said the BBC’s discovery was “yet another example of a Conservative government having to clean up Labour’s mess”.

He added: “We are taking action and have put in place a £3bn plan to improve air quality and reduce harmful emissions as well as ending the sale of new diesel and petrol cars and vans by 2040.”